In mobile communication systems a cell is managed by a base station, BS. Any communication traffic in or out of or within the cell is routed via the BS. The communication is usually sent along a number of channels, each channel assigned to control or data traffic of a particular kind. One example being the Broadcast Control Channel, BCCH, used by a BS to provide a mobile node or Mobile Station, MS, in the cell with control information. Other examples being the Common Control Channel, CCCH, compromising of paging, random access, access grant and notification channels used for control signaling during connection establishment.
At times when many MSs are trying to communicate at the same time in a cell, the control or data traffic can become higher than the BS is capable of handling and the data channels that are used then become congested. In practice this means that some of mobile station's (MSs) traffic will not be handled in a timely manner. Congestion may occur on uplink channels (RACH) or on downlink channels.
To enable devices to communicate freely even under heavy data or control traffic a protocol has been developed wherein a MS has to require a data channel access before starting to communicate. In a system such as the GERAN (GSM EDGE Radio Access Network, GSM—Global System for Mobile communication, EDGE—Enhanced Data rates for GSM Evolution) mobile stations have to send a CHANNEL REQUEST message or an EGPRS CHANNEL REQUEST message on a channel named RACH (Radio Access Channel). If the BS is able to handle the communication a message is sent out granting access on a channel named AGCH (Access Granted CHannel). If the BS is not able to handle the communication a message rejecting the access is sent out on the same channel. The BS may fail to correctly receive the RACH message e.g. if simultaneous RACH messages collide or if the radio link quality is not sufficient. In this case no response is sent to the mobile station at all. Should such a response not be received a MS would resend the request over the RACH. However, during congestions these requests only lead to increased control traffic and therefore adds to the congestion. It should be noted that congestion may occur on either of an upload channel and a download channel or both.
Therefore, it would be beneficial to control the access requests so as not to burden the base stations unnecessarily and thereby alleviate the congestion in a cell.
A procedure during which the MS requests resources for control or data traffic is commonly known as the random access procedure. The MS initiates the random access procedure by transmitting a request message. When the MS sends a request message such as a CHANNEL REQUEST message or an EGPRS CHANNEL REQUEST message on a channel named RACH, the MS is expecting to receive a response from the BS within a given time derived from broadcast parameters. The MS is not allowed to retransmit the request sooner than this time. The reason why the BS does not respond to the request message may be one of the following. Firstly, the request message was not received correctly due to the congestion on a channel named RACH or due to poor link conditions. Secondly, the BS cannot send the response within the time constraints due to the congestion on a channel named AGCH. The random access procedure is delayed in either case by the waiting time between the retransmissions of the request messages.
Therefore, it would be beneficial to estimate the cause of delay in the BS response and allow for shorter waiting time between the retransmissions of the request messages (i.e. faster random access procedure) if the estimation suggests the cause of the delay is an incorrect reception of the request message due to poor link conditions.